The present invention relates to a metal belt-type continuously variable transmission including a metal belt wound around a drive pulley and a driven pulley, and particularly, to a process for controlling the gear shifting speed to obtain a target gear shifting speed in a gear-shifting transient state.
The change in shift ratio in such metal belt-type continuously variable transmission is carried out by applying a smaller axial thrust to one of a drive pulley and a driven pulley, applying a larger axial thrust to the other pulley, increasing the groove width of the one pulley to decrease the radius of winding of the metal belt around the one pulley, and decreasing the groove width of the other pulley to increase the radius of winding of the metal belt around the other pulley. In this case, as the value of the larger axial thrust applied to the other pulley is increased, the groove width of the pulley is decreased quickly to increase the gear shifting speed. Reversely, as the value of the larger axial thrust is decreased, the groove width of the pulley is decreased slowly to decrease the gear shifting speed.
However, it has not been conventionally sufficiently recognized how the gear shifting speed is determined in the gear shifting transient state of the metal belt-type continuously variable transmission. For this reason, an axial thrust required for obtaining a target gear shifting speed could not be predicted at the time of designing. Therefore, there are problems that it is difficult to design a hydraulic pressure system formed to apply an axial thrust to a pulley, that the gear shifting speed of a completed metal belt-type continuously variable transmission does not reach a target value, and that even when the target gear shifting speed is obtained, the efficiency is reduced due to an increase in friction caused by an excessive displacement of an oil pump. To solve these problems, the change in design of the hydraulic pressure system and the experiment must be repeatedly carried out, resulting in a problem that a lot of time and cost are required.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to ensure that the gear shifting speed of the metal belt-type continuously variable transmission can be controlled properly.
To achieve the above object, according to the present invention, there is provided a process for controlling a gear shifting speed in a metal belt-type continuously variable transmission comprising a metal belt comprised of a large number of metal elements carried on metal ring assemblies, the metal belt being wound around a drive pulley and a driven pulley, a first axial thrust being applied to one of the pulleys, and a second axial thrust smaller than the first axial thrust being applied to the other pulley, thereby increasing a radius of winding of the metal belt around the one pulley to which the first axial thrust has been applied, and decreasing a radius of winding of the metal belt around the other pulley to which the second axial thrust has been applied to change a shift ratio, characterized in that the first axial thrust is subjected to a feedforward control based on a target gear shifting speed in a region where the relationship between the first axial thrust and the gear shifting speed defined as a value obtained by differentiating the radius of winding of the metal belt around the one pulley by an amount of tangential movement of the metal belt is maintained substantially linearly.
With the above arrangement, in the region where the relationship between the first axial thrust and the gear shifting speed defined as the value obtained by differentiating the radius of winding of the metal belt around the one pulley by the amount of tangential movement of the metal belt is maintained substantially linearly, the first axial thrust is subjected to the feedforward control based on the target gear shifting speed. Therefore, the gear shifting speed can be controlled to the target value properly and stably. Moreover, the axial thrust required for obtaining the target gear shifting speed based on the relationship between the gear shifting speed and the first axial thrust can be predicted at the time of designing. Therefore, the capacity of the hydraulic pressure system can be suppressed to the bare minimum to avoid the reduction in efficiency and the increase in cost due to the increase in friction.
In addition to the above arrangement, there is provided a process for controlling a gear shifting speed in a metal belt-type continuously variable transmission, wherein the first axial thrust is subjected to a feedback control, so that the gear shifting speed becomes equal to the target gear shifting speed, in regions where the relationship between the gear shifting speed and the first axial thrust is not maintained substantially linearly.
With the above arrangement, the control of the gear shifting speed can be carried out without hindrance in all the regions by carrying out the feedback control in the regions where the feedforward control of the axial thrust is impossible.